Separation of oil and water mixtures is of great importance across a wide range of technologies and industries. For example, oil and water separation problems gained national attention during the 2010 Gulf oil spill and subsequent cleanup efforts. The petroleum industry faces similar water and oil separation challenges as it attempts to extract oil from beneath the sea.
Existing separation devices and methods are either environmentally unfriendly, extremely energy intensive, or incapable of performing the desired separations. For example, in deep sea oil extraction, one energy-intensive approach is to pump oil emulsified in water from the ocean floor to the surface where it is stored it in gravity separation tanks. In addition, once much of the water has been removed from the oil, existing techniques (e.g., ultracentrifugation) are incapable of removing additional, trace amounts of water that remain. These trace amounts of water in oil may cause problems for end users, process equipment, and machinery. In addition, conventional techniques are incapable of removing trace amounts of oil from water—which is a significant environmental concern. Current separation techniques are therefore inefficient and incapable of performing the wide range of oil and water separations of interest.
In recent years, growing environmental concerns have fueled the need for efficient separation of oil-water mixtures. Oil spills, as highlighted by the Deepwater Horizon spills, have lasting detrimental ecological effects. The threat is recurring and persistent; every year over 20,000 oil spills are reported to the U.S. government. Aside from such disasters, fats, oils, and grease are classified as hazardous waste and their removal (e.g., from water before the water being released into the environment) is subject to increasingly more stringent governmental regulation. Generally, according to U.S. regulations, water needs to be cleaned to about 10 ppm of oil or less prior to being discharged.
A variety of techniques have been implemented in the industry, including gravity separation, skimming, and dissolved air flotation. In addition, techniques incorporating aerogels, fluorosurfactant polymers, and magnetic materials have been introduced. However, these approaches have turned out to be ineffective for separation of small-scale emulsions, especially for those with droplets below a micron in size.
Membranes which function primarily on the principle of size exclusion offer cost-effective separation for filtration of solid particles such as protein aggregates and macromolecules. However, it has been found that in the case of emulsified liquids, dispersed droplets can deform and squeeze through pores that are smaller than the emulsified droplet size. To prevent droplets of one phase from squeezing through a membrane, the surface energy of the membrane must be precisely controlled. Thus, separation of nanoemulsions (i.e., oil-water mixtures including sub-micron droplets) remains a key challenge that has not been met by conventional systems. Tradeoffs between geometric constraints, high breakthrough pressure for selectivity, high flux, and mechanical durability make it challenging to design effective membranes for separating nanoemulsions.
For a straight-pore membrane, complete separation of nanoemulsions requires pores smaller than the smallest droplet size. For a given applied pressure, smaller pores lead to a lower flow rate. Decreasing the thickness of the membrane can circumvent this, but extremely thin membranes lack mechanical integrity. In addition, increasing the applied pressure can increase the flow rate, but the maximum pressure must not exceed the breakthrough pressure. Hence, there is a need for new membrane architectures that can achieve both high fluxes and high selectivity while maintaining mechanical durability.
There is a need for more efficient and more environmentally-friendly devices and methods for separating oil-water emulsions (and oil and water mixtures). In particular, a need exists for separating trace amounts of water from oil-water emulsions and for separating trace amounts of oil from oil-water emulsions, including nanoemulsions.